High school math is needed, but I decided to avoid it for as long as I can. It takes about 2 hours to get the whole thing going (and I’ve had to spend my morning, lunch, and breaks on this!) but I end up with a fully functional solution for arbitrary-sized maps and positions. It ought to be fast enough, finding both the solution to part 1 and part 2 in under 100ms combined by doing some fancy filtering.
Source at day10.erl · GitHub
My solution : advent/lib/2019/day10 at master · cblavier/advent · GitHub
Wasted at least 1 hour this morning going totally wrong (I was only dealing with 8 different angles) 
And then I used the high school maths mentioned by @ferd 
Finally made it… I’ve got stuck in part 2 and wasted too much time to understand what I was doing: terrible code had a toll on my brain… (like vague names and hardcoding the starting angle to :math.pi/-2 ).
For few times I went back saying “There should be an easier way…”, re-thinking the problem.. but at the end the way to start (like I think many of you did) was to transform the cartesian coordinates to polar coordinates, grouping the asteroids by polar angle (calculated with math:atan2/2).
The second part takes ~70ms (macbook pro i9), it’s absolutely unoptimized, all coded to get the result and go to sleep 
I’m very happy I found this forum! I just finished day 10 here the code, and the tests but it’s almost the first time I use Elixir and the Phoenix framework so my code can probably be much better
